From the Paragon Wiki:
Any critter that is capable of dropping a level 50 IO has a chance to drop a "purple". This includes, but is not limited to, level 47 minions, level 46 lieutenants, and etcetera. Giant Monsters, Monsters, and Underlings will never drop a "purple".
With that said I specifically remember being told by someone who studies the game far more than I ever did that when setting difficulty make sure to enable Bosses otherwise you won't get purples.
That could just be due to drop rates being lower for minions and LT's but I don't recall ever getting a purple drop with bosses turned off. Again, I don't have supporting evidence (I searched paragon wiki and couldn't find anything to support it's impossible to get a purple drop with bosses turned off) but it's always worked that way for me.
I suspect if you turn bosses on your luck will change